I'm surprised it's legal, and not at all sure it is.

I found this on the site of the Consumer Council for Water:

"Do pubs and restaurants have to supply free tap water?

Yes if they serve alcohol. The government updated rules requiring establishments to have free tap water available wherever alcohol is sold, in an attempt to combat binge drinking. You can read more about the Home Office's latest guidance here. Failure to comply with any conditions attached to a licence or certificate is a criminal offence, which could be punishable by a fine or imprisonment or both. "

The law requiring this evidently came into force on 6th April 2010.

Shop the bastard!